Technically, that means the plane had struck - or been struck by - two birds, Caldwell said.

But Air Line Pilots Association safety committee chairman Rory Kay said the pilot's message could easily have meant that the jetliner had ingested birds in both engines or that it had been struck by more than just two birds.

"It's not easy to count birds," Kay noted wryly, when you're taking off or landing and typical speeds can exceed 100 miles per hour.

She said there haven't been many major accidents due to birds strikes, "not in many years - not like this one. ... It's more common in general aviation - smaller aircraft."

From 1990 to 2007, there were nearly 80,000 reported incidents of birds striking nonmilitary aircraft, about one strike for every 10,000 flights, according to the FAA and the Department of Agriculture.

A commercial airliner like the US Airways plane is most likely to encounter birds on takeoffs and landings because that's when the plane is flying at lower altitudes, Kay said.

Most of the time airliners are flying at 20,000 feet to 30,000 feet where birds are few in number.

But below 5,000 feet is where planes run into trouble, Kay said.

"There is no shortage of bird strike reports ... You just don't get to hear about them," said Kay, a Boeing 767 pilot who has been flying for 34 years.

Kevin Poormon, a senior research engineer at the University of Dayton Research Institute in Dayton, Ohio, tests the ability of airplanes and engines to withstand bird strikes by firing 4 pound to 8 pound birds at strategic points along aircraft from compressed gas guns at hundreds of miles per hour. He said he uses "freshly killed" birds or a gelatin substitute "that has similar impact properties."

If necessary, researchers can launch the birds at up to 900 mph, Poormon said. FAA requires airliners to withstand strikes from birds weighing as much as 8 pounds at particularly vulnerable points along the aircraft, he said.

"It's a pretty significant problem," Poormon said. "There have been over 200 fatalities in the last 20 years due to bird strikes worldwide and there are 5,000 impacts that are reported every year."

"Aircraft are being struck every day by birds," Poormon said. "The reason you don't hear about them so much is they are designed to take these impacts. But once you get to large flocks or large birds striking at a critical moment, that's where these events hit the news."

Richard Dolbeer, a former USDA scientist who coordinated wildlife management at airports nationwide and was chairman of Bird Strike Committee USA, a multi-agency group, said that gulls are a problem nationally along the coasts.

Canada geese living in the United States have increased from about 1 million in 1990 to 3.9 million in 2008, he said.

A resurgence of birds and wildlife that end up being hazardous to aviation has followed the environmental initiatives of the 1960s and 1970s aimed at cleaning up the nation's water, protecting species, eliminating dangerous pesticides, and expanding the wildlife refuge system.

At the same time, aircraft have become faster and quieter so birds don't detect them as readily.

Often, birds are sitting quietly until they hear an airplane coming, and then trying to get away, fly up and wind up striking the plane, Kay said.

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